1. Field of the Invention
The present invention relates to a method of correcting a measured image formed by a television camera.
2. Description of the Prior Art
The inventors of the present invention are engaged in the development of techniques for taking, for example, a plant by a television camera and measuring data representing the activity of the plant, i.e., health barometers of the plant. to evaluate the activity of the plant.
The inventors of the present invention verified through measurement that the reflectance, namely, the ratio in percent of the quantity of reflected light to that of incident light, of natural light on a plant having high activity assumes a characteristic curve as shown in FIG. 10. As shown in FIG. 10, the reflectance decreases with wavelength to a minimum in a wavelength band around a wavelength of 670 nm, and then the reflectance increases sharply with wavelength to a high level in a wavelength band around a wavelength of 850 nm. Such a mode of variation of the reflectance of natural light with wavelength is inferred to be due to the absorption of most part of the incident light rays of wavelengths in an absorption band corresponding to chlorophyll and the reflection of most part of the incident light rays of wavelengths in the near infrared band.
On the other hand, the reflectance of natural light on a plant having low activity assumes a characteristic curve as shown in FIG. 11, in which the reflectance of light rays of wavelengths around a wavelength of 670 nm is slightly higher than that of the plant having high activity, and the reflectance of light rays of wavelengths around a wavelength of 850 nm is far smaller than that of the same on the plant having high activity. Such a mode of variation of reflectance of natural light on the plant having low activity with wavelength is inferred to be due to the slight increase of reflectance to red rays attributable to increase in carotenoid and the significant reduction of reflectance of light rays in the near infrared band attributable to the decomposition of chlorophyll.
The difference in reflective characteristics between the plant having high activity and the plant having low activity can definitely be determined by comparing the spectral reflection ratio IR/R, where R is the spectral reflectance of light rays of wavelengths around 670 nm on the plant having high activity and IR is the spectral reflectance of light rays of wavelengths around 850 nm on the same plant, and the spectral reflection ratio IR/R with the plant having low activity. The spectral reflection ratio IR/R with the plant having high activity is very large because R is relatively small and IR is very large, whereas the spectral reflection ratio IR/R with the plant having low activity is very small because R is relatively large and IR is relatively small. Thus, the activity of a plant can be determined from the spectral reflection ratio IR/R by taking the image of the plant by a television camera.
The amount of energy of incident solar radiation on a plant varies every hour or every season, depending on the solar altitude and conditions of clouds. Since the reflectance of solar radiation on a plant varies with the variation of the amount of energy of solar radiation, image signals provided by a television camera are affected by the variation of the amount of energy of solar radiation; that is, the image signals will vary according to the variation of the amount of energy of solar radiation even if the health condition of the plant remains constant. Accordingly, it is impossible to determine the activity of the plant correctly by simply measuring the spectral reflectance of light rays on the plant and calculating the spectral reflection ratio IR/R.
Generally, the amount of incident light on a television camera needs to be adjusted by adjusting the aperture stop by the diaphragm. The image signals are affected by the adjustment of the aperture stop. Therefore, the spectral reflection ratio obtained by processing the image signals does not represent the activity of the plant correctly if the aperture stop of the television camera is adjusted when producing the image signals.